1. Technical Field
The present application relates in general to vibration control. More specifically, the present application relates to methods and apparatus for isolating mechanical vibrations in structures or bodies that are subject to harmonic or oscillating displacements or forces. The present application is well suited for use in the field of aircraft, in particular, helicopters and other rotary wing aircraft.
2. Description of Related Art
For many years, effort has been directed toward the design of an apparatus for isolating a vibrating body from transmitting its vibrations to another body. Such apparatuses are useful in a variety of technical fields in which it is desirable to isolate the vibration of an oscillating or vibrating device, such as an engine, from the remainder of the structure. Typical vibration isolation and attenuation devices (“isolators”) employ various combinations of the mechanical system elements (springs and mass) to adjust the frequency response characteristics of the overall system to achieve acceptable levels of vibration in the structures of interest in the system. One field in which these isolators find a great deal of use is in aircraft, wherein vibration-isolation systems are utilized to isolate the fuselage or other portions of an aircraft from mechanical vibrations, such as harmonic vibrations, which are associated with the propulsion system, and which arise from the engine, transmission, and propellers or rotors of the aircraft.
Vibration isolators are distinguishable from damping devices in the prior art that are erroneously referred to as “isolators.” A simple force equation for vibration is set forth as follows:F=m{umlaut over (x)}+c{dot over (x)}+kx 
A vibration isolator utilizes inertial forces (m{umlaut over (x)}) to cancel elastic forces (kx). On the other hand, a damping device is concerned with utilizing dissipative effects (c{dot over (x)}) to remove energy from a vibrating system.
One important engineering objective during the design of an aircraft vibration-isolation system is to minimize the length, weight, and overall size including cross-section of the isolation device. This is a primary objective of all engineering efforts relating to aircraft. It is especially important in the design and manufacture of helicopters and other rotary wing aircraft, such as tilt rotor aircraft, which are required to hover against the dead weight of the aircraft, and which are, thus, somewhat constrained in their payload in comparison with fixed-wing aircraft.
Another important engineering objective during the design of vibration-isolation systems is the conservation of the engineering resources that have been expended in the design of other aspects of the aircraft or in the vibration-isolation system. In other words, it is an important industry objective to make incremental improvements in the performance of vibration isolation systems which do not require radical re-engineering or complete redesign of all of the components which are present in the existing vibration-isolation systems.
A marked departure in the field of vibration isolation, particularly as applied to aircraft and helicopters is disclosed in U.S. Pat. No. 4,236,607, titled “Vibration Suppression System,” issued 2 Dec. 1980, to Halwes, et al. (Halwes '607). Halwes '607 is incorporated herein by reference. Halwes '607 discloses a vibration isolator, in which a dense, low-viscosity fluid is used as the “tuning” mass to counterbalance, or cancel, oscillating forces transmitted through the isolator. This isolator employs the principle that the acceleration of an oscillating mass is 180° out of phase with its displacement.
In Halwes '607, it was recognized that the inertial characteristics of a dense, low-viscosity fluid, combined with a hydraulic advantage resulting from a piston arrangement, could harness the out-of-phase acceleration to generate counter-balancing forces to attenuate or cancel vibration. Halwes '607 provided a much more compact, reliable, and efficient isolator than was provided in the prior art. The original dense, low-viscosity fluid contemplated by Halwes '607 was mercury, which is toxic and highly corrosive.
Since Halwes' early invention, much of the effort in this area has been directed toward replacing mercury as a fluid or to varying the dynamic response of a single isolator to attenuate differing vibration modes. An example of the latter is found in U.S. Pat. No. 5,439,082, titled “Hydraulic Inertial Vibration Isolator,” issued 8 Aug. 1995, to McKeown, et al. (McKeown '082). McKeown '082 is incorporated herein by reference.
Several factors affect the performance and characteristics of the Halwes-type isolator, including the density and viscosity of the fluid employed, the relative dimensions of components of the isolator, and the like. One improvement in the design of such isolators is disclosed in U.S. Pat. No. 6,009,983, titled “Method and Apparatus for Improved Isolation,” issued 4 Jan. 2000, to Stamps et al. (Stamps '983). In Stamps '983, a compound radius at the each end of the tuning passage was employed to provide a marked improvement in the performance of the isolator. Stamps '983 is incorporated herein by reference.
Another area of improvement in the design of the Halwes-type isolator has been in an effort directed toward a means for changing the isolator's frequency in order to increase the isolator's effectiveness during operation. One development in the design of such isolators is disclosed in U.S. Pat. No. 5,435,531, titled “Vibration Isolation System,” issued 25 Jul. 1995, to Smith et al. (Smith '531). In Smith '531, an axially extendable sleeve is used in the inner wall of the tuning passage in order to change the length of the tuning passage, thereby changing the isolation frequency. Another development in the design of tunable Halwes-type isolators was disclosed in U.S. Pat. No. 5,704,596, titled “Vibration Isolation System,” issued 6 Jan. 1998, to Smith et al. (Smith '596). In Smith '596, a sleeve is used in the inner wall of the tuning passage in order to change the cross sectional area of the tuning passage itself, thereby changing the isolation frequency during operation. Both Smith '531 and Smith '596 were notable attempts to actively tune the isolator.
Another development in the area of vibration isolation is the tunable vibration isolator disclosed in U.S. Pat. No. 6,695,106, titled “Method and Apparatus for Improved Vibration Isolation,” issued 24 Feb. 2004, to Smith et al, which is hereby incorporated by reference.
Although the foregoing developments represent great strides in the area of vibration isolation, many shortcomings remain.